There are mainly three methods for obtaining white light by using fluorescent powder in a Light-Emitting Diode (LED), but they are not fully mature methods, which severely limits the use of white-light LED in the field of illumination. Specifically, in the first method, yellow fluorescent powder that may be excited by blue light is coated on a blue LED chip, and the blue light emitted by the chip and the yellow light emitted by the fluorescent powder are complementary to each other so as to form a white light. This technology is engrossed by Nichia Company in Japan, and a theoretical disadvantage of such a solution is that the emission spectrum of Ce3+ ions in the fluorophor does not have a continuous spectral characteristic and the color rendering property thereof is poor, thus it is difficult to meet the requirement of low-color temperature illumination, and at the same time, the luminescence efficiency is not high enough, and it needs to be improved by developing novel high-efficiency fluorescent powders.
In the second method, green and red fluorescent powders are coated on a blue LED chip, and the blue light emitted by the chip and the green light and red light emitted by the fluorescent powder are combined so as to obtain a white light, with a good color rendering property. However, the fluorescent powders employed in such a method have a low effective conversion efficiency, especially, the effective conversion efficiency of the red fluorescent powder needs to be improved greatly.
In the third method, fluorescent powders of three primary colors or multiple colors are coated on a violet-light or ultraviolet-light LED chip, and the emission of white light is realized by exciting fluorescent powders via the long-wave ultraviolet light (370 nm-380 nm) or violet light (380 nm-410 nm) emitted by the chip. The color rendering property in this method is better, but a problem similar to that of the second method still exists that the effective conversion efficiency of the fluorescent powders is low; moreover, in the current red and green fluorescent powders which are of a high conversion efficiency usually contains a sulfide system, and such fluorescent powders are of a poor luminescence stability and a large light attenuation.
It is difficult to control the homogeneity of the existing fluorescent powders, so that the luminescence effect of a final luminaire will be influenced. As a result, it has become an urgent task to develop a high-efficiency and low-light attenuation fluorescent powder for a white-light LED.